1. Field of the Invention
The present invention relates to a sheet post-processing apparatus and, more specifically, to a sheet post-processing apparatus which performs a predetermined post-processing operation such as a stapling operation on sheets successively fed therein at a predetermined interval from an image forming apparatus such as a copying machine.
2. Description of Related Art
It is often desired to perform a stapling operation or a punching operation on a relatively large number of sheets each bearing an image transferred thereon by an image forming apparatus such as a copying machine or a printer.
In such a case, a sheet post-processing apparatus or a so-called finisher is conveniently used which automatically performs predetermined post-processing operations including the stapling operation and the punching operation.
An exemplary construction of the finisher is schematically illustrated in FIG. 7. The finisher 100 is detachably connected to a sheet discharging side of an image forming apparatus such as a copying machine, and includes an intermediate tray 101 which is capable of retaining therein a plurality of sheets P. Where a set of sheets successively discharged at a predetermined interval from the image forming apparatus is to be subjected to the stapling operation, the sheets P are once retained in the intermediate tray 101.
An introduction section 102 including introduction rollers 102a and an introduction guide 102b is provided in association with an upper portion of the intermediate tray 101. The sheets P discharged from the image forming apparatus are fed into the intermediate tray 101 via the introduction section 102.
The intermediate tray 101 is provided with a sheet receiver 103 which is movable along the intermediate tray 101. During the introduction of the sheets, the sheet receiver 103 is held on standby at a lower portion of the intermediate tray 101. The sheets P are successively fed from an upper portion of the finisher 100 onto the sheet receiver 103 along the intermediate tray 101 through the introduction section 102. Leading edges of the sheets P are supported by the sheet receiver 103.
A stapler 104 for performing the stapling operation, for example, is provided adjacent a standby position of the sheet receiver 103. The set of sheets P with their leading edges aligned (or registered) on the sheet receiver 103 is subjected to the stapling operation by the stapler 104. The sheet set subjected to the stapling operation is transported upward toward the upper portion of the finisher 100 by the sheet receiver 103, and discharged onto a sheet discharge tray 106 through sheet set discharging rollers 105.
The time required before the intermediate tray 101 is ready for acceptance of the next set of sheets upon completion of the stapling operation performed on the sheet set currently retained in the intermediate tray 101 is longer than the time interval at which the sheets P are successively introduced into the intermediate tray.
Therefore, where sheets P are fed at the predetermined time interval from the image forming apparatus, the first sheet of the next sheet set should be held on standby in a sheet transportation path 107 for some time until the intermediate tray 101 is ready for the acceptance of the next sheet set upon completion of the stapling operation.
In one conventional apparatus (not shown), having a greater length than the ordinary transportation path 107 is provided so that the first sheet of the next sheet set is transported through the longer transportation path and the second sheet of the next sheet set is transported through the ordinary transportation path 107 for elimination of the standby time.
In another conventional apparatus (not shown), two intermediate trays are provided so that, while sheets retained in one of the intermediate trays are subjected to a predetermined post-processing operation, sheets of the next sheet set are transported into the other intermediate tray for elimination of the standby time.
In the case of the first prior art where the second transportation path is provided which is longer than the ordinary transportation path, however, the second transportation path is complicated because of its greater length. Therefore, the apparatus is liable to cause a sheet jam, and has a difficulty in maintenance operations such as a sheet jam recovering operation.
In the case of the second prior art where the two intermediate trays are provided, the apparatus has a correspondingly complicated construction and increased size.
On the other hand, where a positional relationship between the standby position of the sheet receiver 103 and the introduction section 102 is not proper with respect to the length of a sheet P, an event as shown in FIG. 8 may occur. More specifically, a sheet P2 slips into the rear side (the left side as seen in FIG. 8) of a sheet P1 previously fed into the intermediate tray 101.
Particularly, a sheet has a great warpage immediately after being heated for image fixation, so that the slip-in of the sheet is more liable to occur. If the slip-in of the sheet occurs, sheets to be discharged are arranged in a different collating order from an order in which the sheets have been fed from the copying machine.
The sheets transported into the intermediate tray 101 may have different lengths, as measured with respect to the transportation direction, depending upon the sheet type (A size, B size). Therefore, the conventional sheet post-processing apparatus is adapted to variably adjust the positions of the introduction section 102 and the sheet receiver 103 and the standby position of the stapler 104 in accordance with the sheet type. Thus, the positional relationship between the sheet receiver 103 and the introduction section 102 is optimized.
However, driver systems and the like are required for moving the introduction section 102, the stapler 104 and the like in accordance with the sheet type. Therefore, the apparatus has a more complicated construction and a greater size.